Unbleached pulps exhibit a wide range of brightness depending
on the pulping process and fibrous raw material used. The sulfite process
produces the brightest pulp, up to 65% whereas Kraft, soda and semi-chemical may
produced pulp of only 15% brightness.

Holocellulose (Cellulose + hemi-cellulose) are inherently
white and do not contribute to color. It is the chromophoric group in lignin
which are responsible for color.

Two approaches are used in the chemical bleaching of
pulps. One approach utilizes selective chemicals that destroy some of the
chromophoric group but do not attack lignin. The other approach is to almost
totally remove all lignin. The first approach provide around 70% brightness
and retain high pulp yield, while second approach provide 90% + brightness
but reduces pulp yield.

Chlorine dioxide
(ClO2) is a highly selective chemical that can both de-lignify
and brighten pulp. It oxidizes lignin, but does not add chlorine atoms
onto lignin fragments; however, small amounts of elemental chlorine and
other chlorine compounds formed during the chlorine dioxide bleaching
process react with degraded lignin to form chlorinated organic compounds.
For more

Oxygen removes
lignin and modify other coloring components. In the oxygen
delignification/bleaching stage the pulp is treated with oxygen in a
pressurized vessel at elevated temperature in an alkaline environment.
For more

Hydrogen peroxide
(H2O2) is mainly used to brighten pulps in the final
bleaching stages. Peroxide is often used at the end of a conventional
bleaching sequence to prevent the pulp from losing brightness over time.
Preferred for mechanical and recycled fiber.
For more

Ozone (O3) is
also an effective de-lignifying agent. It also brightens the pulp as well.
Ozone attacks the cellulose fiber as well as the lignin.

ZD

Ozone and
Chlorine Dioxide added sequentially in same stage

Bleaching sequences can be any combination of these stages. Lately selection
of bleaching sequence has largely being guided by environmental considerations.
In USA, Canada and Europe, use of elemental chlorine for bleaching is almost
stopped. Two categories of bleaching sequences used are; ECF (Elemental Chlorine
Free) and TCF (Total Chlorine Free). ECF uses Chlorine Dioxide while TCF exclude
Chlorine Dioxide. Read more about ECF from
Alliance for
Environmental Technology, a group lobbying for ECF.

Type of Pulp: The high yield pulp which retain most of their lignin
are bleached by modifying the lignin instead of removing it. Chemical pulp
where most of the lignin is already removed bleaching is performed by
removing the remaining lignin. Recycled fiber are de-inked before any
bleaching.

Degree of Brightness: The bleaching sequence and number of stages
are mostly governed by the requirement of brightness
levels. A simple CEH or DEH is enough to achieve 70 or below brightness. 4
to 5 stages are required for 80 -90 brightness and more stages for >90
brightness.